This invention relates to a thermally bonded nonwoven fabric and, more particularly, to a thermally bonded nonwoven fabric endowed with both high nonwoven fabric tenacity and soft hand formed of composite fibers excellent in spinnability, stretchability and also excellent in fusion bonding characteristics during formation into nonwoven fabric.
Nonwoven fabrics obtained by use of composite fibers comprising constituents with different melting points have been prepared by a process which comprises making composite fibers of side-by-side type or sheath/core type from a higher melting resin component and a lower melting resin component, followed by thermal bonding thereof, as disclosed formerly in Japanese Patent Publication Nos. 22547/1969 and 12380/1977, which process has become the basic technique for producing nonwoven fabric products which are used as constituent materials for disposable type diapers, sanitary napkins and the like which have been rapidly growing in the market in recent years. In such process, as the lower melting resin component, high density polyethylene, conventional branched type low density polyethylene, ethylene-vinyl acetate copolymer, atactic polypropylene, polybutene, etc. have been used, while as the higher melting resin component, isotactic polypropylene, polyester, polyamide, etc. have been used.
However, with abrupt growth of nonwoven fabric products in the market, it has become a great demand to have a product which is excellent in spinnability and capable of producing filaments of small denier stably and continuously in carrying out spinning and capable of effecting uniform thermal bonding at a lower temperature and within a shorter time in heat treatment for forming a nonwoven fabric, and is also endowed with both high nonwoven fabric tenacity and soft hand when formed into a nonwoven fabric. For exhibiting soft hand, the spun filament is required to be of small denier. In this regard, soft resins having a long chain branching such as conventional branched type low density polyethylene and ethylene-vinyl acetate copolymer which are to be used as lower melting components, have generally high elongational viscosity and therefore are susceptible to scission during spinning, whereby drawing ratio cannot be increased. Thus fibers of small denier can be prepared with difficulty. Accordingly, high density polyethylene entailing fewer such problems with relatively good fiber forming property has been employed primarily as the lower melting component, and compositely spun together with polypropylene or the like etc. As the high melting component into fibers of side-by-side type or sheath/core type. However, high density polyethylene is insufficient in fusion bonding characteristic for the level now demanded and inferior in productivity due to high temperature and long time required for heat treatment. Besides, for exhibiting soft hand, it is preferable to use fibers of the smallest possible fineness and make specific volume higher in nonwoven fabric formation to make the unit weight lower, and hence there is a tendency that the effectively bonded area or cross-over point number of the fiber cross-over points responsible for the strength of nonwoven fabric is reduced. Recently, an ethylene-.alpha.-olefin copolymer represented by linear low density polyethylene which has approximately comparable spinnability as compared with high density polyethylene, is attracting attention as the resin meltable at lower temperature, but no sufficiently satisfactory resin responding to the above demands can be found yet.
On the other hand, spinning of thermally bondable composite fibers is ordinarily practiced at a temperature higher than the melting point of the higher melting component as a matter of course, and practically at a considerably high temperature of 250.degree. C. to 350.degree. C., because the melt viscosities of the respective components are required to be adequately controlled for forming sheath/core forms at the fiber sections. Therefore, in case of ethylene-.alpha.-olefin copolymers which have more short chain branchings, with greater extrusion resistance and larger tendency to generate heat by shearing as compared with high density polyethylene, the molecular structure change by crosslinking deterioration during spinning may become a very serious problem. This can be estimated, for example, from a great change in melt flow ratio before and after spinning. Such molecular structure change has detrimental effect on fusion bonding characteristic through formation of oxidized skin on the fiber surface, in addition to causing lowering of continuous running performance by fluctuation of the filaments during spinning due to thermal decomposition or by frequent occurrence of cutting troubles of filaments due to gel generation caused by molecular crosslinking. As a countermeasure against these troubles, since the conditions such as spinning temperature, etc. can be changed with difficulty, sufficient preventive recipe against oxidation deterioration must be applied on the resin, and also in this respect, said material has not yet been sufficiently investigated.
As stated above, though it has been expected that some of ethylene-.alpha.-olefin copolymers would have more suitable characteristics as the lower melting resin component of thermally bondable composite fibers than high density polyethylene, there has been obtained no product yet which is sufficiently satisfactory as the thermally bonded nonwoven fabric endowed with both high nonwoven fabric tenacity and soft hand.